As conventional techniques for preparing an ultra fine fiber or nano fiber suitable for the production of artificial leather, there are known a sea-island type conjugated spinning method, a division type conjugated spinning method, a blend spinning method and so on.
However, in case of the sea-island type conjugated spinning method or the blend spinning method, one of two polymer components comprising a fiber must be dissolved and removed for making the ultra fine fiber. In order to produce artificial leather from the fiber prepared by these methods, a complex process must be carried out, including melt spinning, nano fiber production, non-woven fabric production, urethane impregnation and single component dissolution. Nevertheless, it has been impossible to produce a fiber with a diameter 1,000 nm or less by the above two methods.
In case of the spit type conjugate spinning method, it has been problematic in that since two polymer components (for example, polyester and polyamide) with different dyeing properties co-exist in a fiber, uneven dyeing occurs and an artificial leather production process is complicated. In addition, it has been difficult to produce a fiber with a diameter 2,000 nm or less by the above method.
As another conventional technique for preparing a nano fiber, an electrospinning method is suggested in U.S. Pat. No. 4,323,525.
In the electrospinning method, a polymer spinning dope in a spinning dope main tank is continuously and constantly fed into a plurality of nozzles, which has a high voltage applied, through a metering pump. Subsequently, the spinning dope fed to the nozzles is spun and collected through the nozzles on a collector of an endless belt type having a high voltage more than 5 kV, thereby producing a fiber web.
The conventional electrospinning method can produce only a web or non-woven fabric composed of a nano fiber 1,000 nm or less. Thus, it is difficult to prepare a continuous filament using the conventional electrospinning method. Hence, to prepare a continuous filament, the produced nano fiber web has to be cut to a predetermined length to produce a staple fiber and this staple fiber has to be blown and undergone an additional spinning process, which makes the process complicated.
A spinning distance (distance between the nozzle and the collector) is so short in an electrospinning process that a method capable of drawing by applying a physical force is restrictive, and thus the mechanical properties are very low.
Meanwhile, as a method for arranging nano fibers in a fiber axis direction when preparing a filament composed of nano fibers, it has been already explained that fibers are arranged between conductive lines by placing the conductive lines on both sides of a nonconductive material such as quartz and then performing electrospinning thereon [Dan Li, Yuliang Wang, and Younan Xia, Advanced Materials Vol 16(4), pp 361-366, 2004]. However, this method has a low possibility of industrialization, and any drawing force cannot be applied to this method.
Meanwhile, Korean Patent Application No. 2004-6402 discloses a process of preparing a filament composed of a nano fiber by preparing a ribbon-like nano fiber web by electrically spinning a nano fiber on a roller, twisting it while passing it through an air twisting machine, and then drawing it. However, this conventional process is problematic in that the strength of the prepared filament is low due to poor arrangement of nano fibers in the fiber axis direction.
As seen from above, there is a problem that it is not possible to mass-produce a continuous filament composed of a nano fiber which is superior in drawing properties due to poor arrangement of nano fibers in the fiber axis direction by the conventional techniques known so far.